The response to hormones is studied in an epithelium as it forms in culture. Epithelia formed by A6 cells, derived from kidney of Xenopus laevis, do not respond to vasopressin when grown on a petri dish. When grown on a millipore filter bottom cup, the epithelia respond to vasopressin. After seeding on a filter, the responsiveness of each subunit of adenylate cyclase was evaluated over a period of two weeks. cAMP accumulation was measured in response to stimulation of hormone receptor (by vasopressin), G nucleotide binding regulatory subunit (by chorela toxin), and catalytic subunit of adenylate cyclase (by forskolin). Evaluation was also performed by assays of adenylate cyclase activity in membrane rich fractions prepared from homogenates of the epithelia. The activity of each subunit appears to develop with a different time course. In addition, there is a close correlation between the development of vasopressin sensitive adenylate cyclase, transepithelial electrical resistance, and formation of a morphologically ordered epithelium.